Marketing Automation is the act of setting up elements of your marketing process to happen automatically, without the need for manual input. Running the gamut from sending a thank you email when a lead submits a form, to updating data in your CRM based on user-action, Marketing Automation is always a huge time-saver for a business.
Often mistakenly considered to just be the automatic sending of emails, general marketing automation can be a significantly deeper, more useful tool. Any action that happens multiple times for the same reason can be automated - an email being sent, a task being issued, data being updated etc. If your work day has ever consisted of doing the same boring activity time and time again, it can probably be automated for you.
If you'd like to work in a business with a system that can manage and respond to your users while you sit back and focus on the important stuff (like sweeping all those dang mines for example), then marketing automation is what you need.